This invention relates to a process for completely and substantially uniformly coating the surfaces of a polymeric monofilament or a yarn made from a plurality of polymeric monofilaments with electrolessly deposited nickel and optionally with electrolytically deposited metal on the nickel, and to the thermally stable composite obtained by the process.
At the present time, it is difficult to deposit metal coatings onto polymeric monofilaments or multifilament yarns to form composite products which are thermally stable and/or deposit metal coatings which not easily removed from the monofilament or yarn by low or moderate frictional forces. It has been proposed to coat polymeric fibers with electrolessly deposited copper followed by electrolytically deposited copper. When subjected to thermal cycling tests, however, these coatings are unstable in that they crack and lose metal adhesion.
In order to provide a commercially viable process for metal coating yarns, a continuous process rather than a batch process must be provided. In such a process, yarn to be treated is unwound from a feed storage reel, passed through the appropriate chemical treating steps and then stored on a take up reel. Unfortunately, in presently available yarn processing means, the monofilaments positioned within the interior of the yarn are not coated or are insufficiently coated so that the metal coatings on the monofilaments are non-uniform. A non-uniformly coated yarn has undesirable non-uniform electrical conductivity. In many applications, such as for protective outside layers for coaxial cables, non-uniform metal outside layers are unacceptable.
Polymeric fibers are generally made in the form of bundles of monofilaments called yarns. They are extruded through spineret nozzles having a plurality of holes that control the number of filaments and their diameters. The monofilaments from a given extrusion nozzle are gathered into a single yarn bundle (tow or roving). The diameter of the monofilaments of typical fibers used for weaving into cloth or other useful textile articles are generally in the range of 10-15 micrometers. The size of the yarn bundle is generally designated by the term, denier. Which is the weight, in grams, of 9,000 meters of yarn. Commercial yarns generally range in size from very fine denier up to a very thick string or rope-like consistency of 5,000 denier. Typically, 55-3,000 denier are used for most applications. The number of filaments in a given denier yarn will vary with the density and weight of the polymer forming the filaments but are generally in the same range. For example, with the polyaramid, Kevlar.RTM., a 55 denier yarn contains 24 monofilaments which are 14 micrometers in diameter; a 200 denier yarn contains 89 of these monofilaments; a 400 denier yarn contains 178 of these filaments; and a 3,000 denier yarn contains 1,333 of these monofilaments. There is a generally linear relationship between the number of monofilaments per yarn strand and the denier, as well as the basis weight of the yarn, generally expressed in mg/ft. Yarns formed from a multiplicity of aggregated smaller monofilament polymeric fibers are capable of withstanding tensile forces so that the yarn remains intact under tension. The degree of tensile forces which a given yarn can withstand depends upon the number of monofilaments forming the yarn and upon the type of polymeric composition forming the fibers. For example, polyaramid monofilament, (e.g. Kevlar.RTM.) is capable of forming a light weight yarn able to withstand very high tensile forces.
As the content and complexity of electronic equipment installed in military and commercial aircraft has increased over the years, the space/weight devoted to interconnect cabling has likewise increased, along with the need to ensure signal transmission integrity at ever higher frequencies. Interconnect system designers are therefore presented with a challenging, if not contradictory set of requirements: on the one hand, high frequency transmission lines must employ coaxial shielding to ensure signal transmission integrity and to suppress electromagnetic interference (EMI); on the other hand, the shielding in question, typically a braided wire jacket applied in the cable-making process, adds weight and inflexibility to the cables. One obvious approach to this problem is to use smaller diameter wire for the jacketing. Unfortunately, finer gauge copper alloy wire does not have the mechanical strength to reliably withstand the tensions imparted to the wire in the braiding process or in environments such as an aircraft environment where vibration and shock stresses are routine. As a result, designers of coaxial shielding for aircraft interconnect cables are obliged to use wires that are larger and heavier than they need to be if only the electrical requirements of the application are taken into account.
It has been extremely difficult to deposit electroless nickel uniformly and completely onto the surface of monofilaments in a multifiliament yarn bundle by wet chemical electroless processes. Various types of pre-woven fabric are coated with electroless metal, primarily electroless copper, for use as electromagnetic interference (EMI) control and shielding. However, electroless copper, although appearing to have adequate adhesion to the individual monofilament polymer surface of a pre-woven fabric, will not maintain its adhesion after exposure to high temperature or humidity exposure. The copper oxidizes at the polymer-metal interface due to diffusion of entrained moisture in the polymer or oxygen migration which causes a loss in bond integrity. This problem is alleviated by using electroless nickel which forms tight polymeric bonds to the various functional groups on the surface of treated polymers. The resultant nickel-coated filaments are resistant to degradation during exposure to thermal cycling and humidity. Even though pre-woven polyaramid cloth has been previously plated with electroless metals such as copper and nickel, as disclosed in U.S. Pat. No. 4,522,889, complete and substantially uniform metal coatings have not been achieved, particularly at the yarn crossovers of the prewoven fabrics where uncoated filaments commonly occur.
U.S. Pat. No. 5,302,415 describes a process for electrolessely metalizing various polyaramid fibers using copper, nickel, silver, or cobalt. The disclosed process utilizes an 80 to 90% sulfuric acid solution to modify the surfaces of the polyaramid fibers. Modification is achieved by controlled fiber degradation as a consequence of depolymerization, to provide sites for the deposition of a sensitizer which promotes electroless metal deposition. An all-electroless copper construction is undesirable for this application in several respects. The deposition of copper by wet chemistry means deposition directly onto polymer surfaces is undesirable for the reasons set forth above. Its strength of adhesion is extremely weak after thermal cycling accelerates the growth of this copper oxide layer and which eventually leads to interfacial bond failure, i.e., delamination copper from the polymer surface. This phenomenon explains the substantial increase in resistance that all of the electroless copper examples in Table 3 of the referenced patent displayed after exposure to elevated temperature cycling. Resistance changes of this magnitude (4-5 times) are unacceptable for electronic applications, particularly for coaxial shielding. Moreover, the deposition of electroless copper typically produces a coarse-grained metalization which lacks the ductility and flexural endurance that the coaxial cable shielding applications in question require. Furthermore, an all-electroless copper construction would require the addition of another metal layer on each monofilament to protect the exposed copper against long-term oxidation/corrosion. The referenced patent also suggests the alternative use of an all-electroless nickel metalization. No data is supplied in the referenced patent to support the proposed use of an all-electroless nickel process to provide fiber with a metalized surface comparably conductive to copper. However, it is well known that conventional phosphite-reduced electroless nickel processes deposit a layer of metalization having a conductivity typically less than 15% that of copper. Due to oxidation of the phosphorous in the nickel-phosphorous alloy, such deposition form a much more stable surface and are generally preferred for applications involving high corrosion resistance. However, they are highly resistive and difficult to clean (deoxide). Thus, it is difficult to electroplate other metals on these nickel-phosphorous layers, especially as thin coatings on polymeric filaments. Thus, an all-electroless nickel based on conventional phosphite-reduced chemistry is poorly-suited to the goal of achieving a metalized fiber coating with a high conductivity to weight/thickness aspect.
Accordingly it would be desirable to provide polymeric yarn which is completely and substantially uniformly coated with a metal. It would also be desirable to provide such a completely coated yarn capable of having high conductivity to weight/thickness aspect. In addition it would be desirable to provide such a metal coated yarn which can be formed by continuous reel-to-reel process. Such a process would permit the commercial production of completely and substantially uniformly metal coated yarn that could be utilized in a wide variety of environments such as EMI shielding.